Apparatus and method for forming shaped articles

ABSTRACT

A method and apparatus for forming shaped articles such as heat exchanger plates and fuel cell plates from a strip of material. The apparatus includes a feed mechanism for feeding the strip; one or more forming devices which form peripheral features and/or discontinuities in the strip; and an alignment device which maintains constant spacing between the discontinuities and thereby avoids accumulation of incremental feeding errors. Preferably, the strip is re-aligned once for each plate produced. The alignment device comprises a pilot pin and a pilot hole punch for forming index holes in the strip, and can be integral with or separate from the forming devices. One preferred apparatus utilizes a forming device comprising a plurality of punches mounted on a reciprocating frame or gantry structure.

FIELD OF THE INVENTION

The invention relates to apparatus and methods for production of shaped articles, more particularly to apparatus and methods for performing a number of stamping and/or punching operations on a strip of material.

BACKGROUND OF THE INVENTION

Heat exchangers are commonly formed from stacks of flat plates. In such heat exchangers, spaces between adjacent plates define internal flow passages. Heat exchangers of this type are described in U.S. Pat. No. 5,794,691 (Evans et al.) and U.S. Pat. No. 6,244,334 (Wu et al.). This type of stacked plate construction is also used in fuel cells and electrolyzers. In these applications, individual plates (bipolar plates) may be bonded together in pairs containing spaced apart passages for coolant flow between them, and outer facing passages for reactant fluid flow. The bipolar plate pairs are then assembled into a stack comprising catalyst-coated membranes and gas diffusion layers sandwiched between each of the bipolar plate pairs. The plates described herein are typically metal, although other materials such as conductive composite polymer materials may also be used.

For metallic constructions, each individual plate in a stacked plate heat exchanger or fuel cell may be formed by a plurality of separate punching and/or stamping operations performed by one or more different devices. For example, the plates can be formed from a strip by punching apertures, bosses and/or other discontinuities in a central portion of the plate and then passing the strip through a progressive stamping die to form the periphery of the plate and to separate it from the strip. The strip may be unwound from a coil and incrementally fed to the punching apparatus by a feed mechanism having one or more feed rollers.

In such operations it is difficult to ensure that the discontinuities formed by the various punching operations are properly spaced from each other and from the periphery of the plate. In particular, small errors in feeding the strip are repeated for each incremental advance of the strip. These errors are compounded during the course of a production cycle and may result in parts which do not meet specifications and must be scrapped or may contribute to small incremental assembly errors that are initially undetected but can result in decreased product yield in the downstream assembly process, resulting in even greater losses.

Moreover, traditional progressive dies can be extremely expensive for plates that require forming sections to produce the intricate patterns of formed channels and bosses typically required for fluid passages in the intended product applications. Finally, in some heat exchanger constructions and most fuel cell constructions, optimum functional design requires dissimilar plates to be joined alternately in the assembled stack. Such dissimilar plates often have common peripheral features for bonding but have different form sections (different channel patterns or channel heights), or different manifold boss or other internal feature locations or heights. In the case of fuel cell constructions for instance, separate anode and cathode plates are required, which may require substantially different geometric features and channel heights. To produce products of this type by conventional means, two different progressive dies must be purchased at a substantial cost increment. Moreover to maintain balanced production flow in a continuous production setting, the two dies would need to be run in parallel on two presses, further compounding the cost and complexity of operation. To date, it has not been possible to produce different types of plates from a single strip of material, in a mass production environment, without using complex and expensive manufacturing equipment as described above.

The need exists for apparatus and methods which are able to accurately perform a plurality of punching and/or stamping operations and which permit efficient production of different types of plates from a single strip of material, preferably using low cost tooling equipment which allows the production of different or alternating types of parts in a balanced production flow process.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides an apparatus for forming a plurality of shaped articles from a strip of material, the apparatus comprising: a feed mechanism for incrementally feeding the strip of material in a forward direction along a strip feed axis; a forming device which receives the strip of material from the feed mechanism and which forms discontinuities in the strip of material at axially-spaced intervals; an alignment device for maintaining constant axial spacing between the discontinuities, the alignment device being located in fixed axial relation to both the forming device and the feed mechanism, the alignment device comprising a frame structure having a first portion and a second portion between which the strip is fed, the first and second portions being movable toward and away from one another, a pilot hole punch for forming pilot holes in the strip and a pilot pin rigidly mounted in axially-spaced relation to one another on the first portion of the frame structure, the pilot pin being spaced forward of the pilot hole punch so that the pilot pin is insertable into a pilot hole previously formed by the punch; wherein the pilot pin and the pilot hole punch each have a length such that when the first and second portions of the frame structure are moved toward one another, the pilot pin passes through a pilot hole before the punch contacts the strip and aligns the strip relative to the pilot hole punch and the forming device.

In another aspect, the present invention provides a method for forming a plurality of shaped articles from a strip of material, comprising the following steps: incrementally feeding the strip of material in a forward direction along a strip feed axis; forming discontinuities in the strip of material at axially-spaced intervals; periodically aligning the strip to maintain constant axial spacing between the discontinuities by use of an alignment device comprising a frame structure having a first portion and a second portion between which the strip is fed, the first and second portions being movable toward and away from one another, the alignment device including a pilot hole punch and a pilot pin rigidly mounted in axially-spaced relation to one another on the first portion of the frame structure; after each said incremental feeding step, moving the first and second portions of the frame structure toward one another, thereby causing the pilot hole punch to punch a pilot hole in the strip and to simultaneously cause the pilot pin to pass through a pilot hole previously formed by the pilot hole punch; wherein the pilot pin and the pilot hole punch each have a length such that, during movement of the first and second portions of the frame structure toward one another, the pilot pin passes through a pilot hole before the punch contacts the strip, and the pilot pin thereby aligns the strip relative to the pilot hole punch.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, with reference to the accompanying drawings in which:

FIG. 1 is a perspective view of an apparatus according to a first preferred embodiment of the invention;

FIG. 2 is a side elevation view of the apparatus of FIG. 1;

FIGS. 3 to 10 are schematic side elevations showing a preferred alignment device according to the invention; and

FIG. 11 is a perspective view of stamped strip stock produced using a preferred method and apparatus according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1 and 2 illustrate a preferred apparatus according to the present invention. Apparatus 10 unwinds a flat strip 12 of a material such as sheet metal from a coil 14 and converts it through a plurality of punching and stamping operations to a shaped article which, in the drawings, comprises a dished plate 16 for a heat exchanger. Examples of patents describing dished plate heat exchangers include U.S. Pat. No. 5,291,945 (Blomgren et al.) and U.S. Pat. No. 6,182,746 (Wiese et al.).

As shown in FIG. 11, the plate 16 has a periphery 18 provided with a side wall 20 and a central portion 22. The central portion 22 is provided with one or more discontinuities which, in the context of dished plate production, are typically selected from one or more members of the group comprising ribs, dimples, holes and raised apertured bosses. These discontinuities are typically formed by punching, stamping or drawing the sheet metal strip 12 using one or more punches and/or dies. It will, however, be appreciated that other types of discontinuities can be formed in the strip 12. For example, discontinuities can be formed by welding other components onto the strip or by cutting or bending the strip.

In the preferred embodiment shown in the drawings, the central portion 22 of plate 16 is provided with two holes 24 and two raised apertured bosses 26. A heat exchanger (not shown) can be formed by stacking plates 16 one on top of each other in a conventional manner, as described in the Blomgren et al. and Wiese et al. patents mentioned above.

The apparatus 10 further comprises a feed mechanism 28 for incrementally feeding the strip 12 of material in a forward direction (as shown by arrows A and B) along a strip feed axis S. The feed mechanism 28 is schematically illustrated in the drawings as comprising an upper feed roller 30 and a lower feed roller 32 between which the strip 12 passes. The feed rollers 30, 32 are mounted in a frame member 33 which is provided with an engagement mechanism 34 for moving the rollers 30 and 32 toward and away from one another in the directions indicated by double-ended arrow C.

As would be apparent to one skilled in the art, the strip 12 is incrementally advanced in the forward direction (arrow B) when engaged along its top and bottom surfaces by the upper and lower rollers 30, 32 and with rollers 30,32 rotating as indicated in FIGS. 3, 5 and 10. Similarly, the strip 12 is released from engagement with feed mechanism 28 when rollers 30, 32 are moved away from one another.

As it is unwound from roller 14 by the feed mechanism 28, the strip 12 is fed axially along a feed table 36 to a first forming device 38 which forms discontinuities in the strip 12 of material at axially-spaced intervals. Preferably, the first forming device comprises a frame structure 40 having an upper arm 41 and a lower arm 42 which are substantially parallel to one another and are connected together at their ends by a pair of side members 47, 49. The frame structure 40 is supported to reciprocate laterally of the strip feed axis S (in the directions indicated by arrows D and E). As shown in FIG. 1, the lower arm 42 of frame structure 40 may preferably be supported by a stationary beam 43 along which it can be moved laterally. A reciprocating drive (not shown) is provided for reciprocating the frame structure 40 laterally of the strip feed axis S along beam 43.

A plurality of tools for forming discontinuities in the strip 12 are supported on the upper arm 41 of the frame structure 40. In the preferred embodiment shown in the drawings, in which the discontinuities comprise holes 24 and bosses 26, the tools comprise punches 44. A total of four punches 44 are illustrated in the drawings. However, it will be appreciated that more or fewer punches 44 may be required, depending on the form and orientation of the discontinuities in the article 16. In the embodiment shown in the drawings, one of the punches 44 is for forming the holes 24 while the other three punches 44 are for forming and shaping the bosses 26.

Each of the punches 44 is actuated by a punch actuator mechanism 45 which, in the preferred embodiment shown in the drawings, comprises a pneumatic cylinder 46 having a piston 48 pivotably connected to a bell crank 50. As shown in FIG. 2, each bell crank 50 is pivotable about a fixed point 51 and is connected to its associated punch 44 through a pivoting connecting arm 52.

Located directly under the frame structure 40 and under strip 12 is a lower die block 53 which is provided with a plurality of die buttons 54, each of which mates with one of the punches 44.

As will be appreciated, the provision of multiple punches 44 on the transversely reciprocating frame structure 40 provides great flexibility and enables the formation of various configurations of discontinuities in the central portions 22 of plates 16. For example, a controller (not shown) may be programmed so that the punches 44 form different configurations of discontinuities in alternating plates 16. Therefore, the apparatus of the present invention conveniently permits the production of two or more different plates from a single strip 12 of material.

In addition to forming different configurations of discontinuities on alternating plates, the apparatus according to the invention also has the ability to produce discontinuities of different heights on alternating plates. This is advantageous, for example, in the manufacture of anode and cathode plates for use in fuel cells. Typically, the anode and cathode plates are provided with different rib patterns and channels of different height. The provision of different channel heights in alternating plates can be accomplished by several methods, including the use of a programmable, variable stroke press or by using an additional die on alternating plates to provide greater channel height.

The apparatus according to the invention may also include one or more additional forming devices. In the preferred embodiment shown in the drawings, a second forming device 56 is provided. Forming device 56 preferably comprises a stamping apparatus and, as shown in the drawings, more preferably comprises a progressive stamping die for forming the peripheries 18 of plates 16 and for separating the plates 16 from the strip 12. The second forming device 56 includes an upper stamping die 58 and a lower stamping die 60. Where the second forming device 56 comprises a progressive stamping apparatus, the dies 58, 60 preferably comprise a plurality of stamping stations 62, 64, 66 and 68 in which features of the periphery 18 and/or central portion 22 of plate 16 are progressively stamped. It will be appreciated that the number of stamping stations in the progressive stamping apparatus may vary from that shown in the drawings.

The second forming device includes a mechanism 78 for opening and closing the dies 58, 60 in the directions indicated by double-ended arrow F (FIG. 1). As shown in the drawings, the mechanism 78 may comprise hydraulic cylinders 80 having pistons 82 which act on a block 84 carrying the upper stamping die 58.

The apparatus according to the invention also comprises an alignment device 85 for maintaining constant axial spacing between the discontinuities formed in strip 12. In order to maintain constant spacing, the alignment device 85 is located in fixed axial relation to the first and second forming devices 38, 56 and to the feed mechanism 28.

The alignment device 85 is schematically illustrated in the drawings as comprising an upper die 86 and a lower die 88 between which the strip 12 is fed. The dies 86 and 88 are mounted in a frame member 118 which is provided with a hydraulic cylinder 120 or similar means to open and close the dies. 86 and 88. As shown in FIGS. 3 to 10, the upper and lower dies are movable toward and away from one another.

As shown in FIG. 3, the upper die 86 of the alignment device 85 may preferably comprise a pair of die plates, including a first die plate 90 and a second die plate 92 biased apart by a coil spring 94 or other suitable biasing means. The first die plate 90 carries a pilot hole punch 96 and a pilot pin 98 and the second die plate 92 is provided with apertures 100 and 102 through which the pilot hole punch 96 and pilot pin 98 can move reciprocally as described below.

FIG. 3 illustrates the first step of a production cycle, in which an end of the strip-12 is fed by rollers 30, 32 to the first forming device 38. As illustrated in FIG. 3, when the rollers 30, 32 of feed mechanism 28 engage the strip 12 and are rotated as indicated by arrows H and 1, the strip 12 is advanced by an incremental amount in the direction shown by arrow G. The forward end of the strip becomes received between the upper and lower dies 86, 88 of the alignment device 85. The upper and lower dies 86, 88 are open to permit feeding of the strip.

As shown in FIG. 4, rotation of the rollers 30, 32 is discontinued in order to stop incremental feeding of the strip 12, but the rollers 30, 32 remain in engagement with the strip 12. The upper die 86 of the alignment device 85 is then brought down to punch a pilot hole 104 in the strip 12. The plug 106 removed by the punch 96 is preferably ejected from the apparatus 85. As shown in FIG. 4, the pilot hole punch 96 preferably extends into an aperture 108 in the lower die 88 when the upper and lower dies 86 and 88 are closed.

Following formation of the pilot hole 104, the dies 86 and 88 are opened and rollers 30 and 32 are again actuated to cause forward feeding of the strip by an incremental amount as shown in FIG. 5. Feeding of the strip 12 is discontinued once the strip reaches the position shown in FIG. 6.

As can be seen from FIG. 6, the incremental amount by which strip 12 is fed corresponds to a distance between the pilot hole punch and the pilot pin, causing the pilot hole 104 to become aligned with the pilot pin 98. However, due to feeding errors, the pilot hole 104 may not be precisely aligned with the pilot pin 98. The magnitude of the misalignment shown in FIG. 6 may be exaggerated. In FIG. 6, the rollers 30,32 have advanced the strip 12 slightly too far forward. If left uncorrected, these feeding errors would lead to inaccuracies in the locations of the discontinuities which would be compounded over the course of a production cycle.

The strip 12 is brought back into alignment as shown in FIGS. 7 and 8. First, rollers 30, 32 are disengaged from the strip 12, for example by withdrawing one or both rollers 30,32 away from the strip 12. FIG. 7 shows roller 30 being withdrawn in an upward direction in order to disengage the strip 12. During or after disengagement of the strip from rollers 30, 32, the upper and lower dies 86 and 88 of the alignment device are again closed. As can be seen in FIG. 7, the relative lengths of the pilot pin 98 and the pilot hole punch 96 are such that when the upper and lower dies 86, 88 of the alignment device are moved toward one another, the pilot pin contacts the strip 12 first and passes though the previously formed pilot hole 104 before the punch 96 contacts the strip 12. The pilot pin 98 has a rounded tip 99 which contacts the edge of the pilot hole 104, causing movement of the strip until it is precisely aligned with the pin 98. The pin 98 then passes through the pilot hole 104 to be closely received inside an aperture 110 in the lower die 88 of alignment device 85. As will be appreciated, the disengagement of feed mechanism 28 permits the strip to be moved by a small distance either backward as shown in FIG. 7, forward or laterally. In order to provide effective alignment, the pilot hole punch 96 and pilot pin 98 are preferably of substantially the same diameter.

After the strip 12 is aligned as shown in FIG. 8, the upper and lower dies 86 and 88 continue to close and the pilot hole punch 96 is then brought into contact with the strip 12 so that a second pilot hole 104′ is formed in the strip 12 and a second plug 106′ is ejected from the device 85. It will be appreciated that the distance between pilot holes 104 and 104′ corresponds exactly to the distance between the pilot hole punch 96 and the pilot pin 98. Following formation of the second pilot hole 104′, the upper and lower dies 86, 88 of device 85 are open and the rollers 30, 32 of feed mechanism 28 are again brought into engagement with the strip for further incremental movement. It will also be appreciated that the rollers 30, 32 may be brought into engagement with the strip 12 before opening of the dies 86 and 88. For example, the rollers 30, 32 can be brought into engagement with strip 12 during or immediately after formation of pilot hole 104′, and immediately after alignment of the strip 12 by the pilot pin 98.

The steps illustrated in FIGS. 6 to 10 are then repeated for each incremental movement of the strip 12, such that a series of precisely aligned pilot holes 104 are formed along the length of the strip 12. The pilot holes 104 can be seen in the strip 12 shown in FIG. 11. As illustrated, pairs of pilot holes 104 are preferably formed at regularly spaced intervals along opposite edges of the strip 12.

The alignment device 85 may be located at any one of a number of different locations along the strip feed axis S. In the preferred embodiment shown in FIGS. 1 and 2, the alignment device 85 is located between the feed mechanism 28 and the first forming device 38 and is mounted on the same base 112 as the first forming device 38. In the alternative, the alignment device 85 may preferably be located between the first and second forming devices 38, 56 or it may be incorporated into either the first or second forming device 56. For example, where the second forming device is a progressive stamping apparatus, the alignment device 85 may preferably be located at a first stamping station of the apparatus. The alignment device 85 may also comprise a self-contained unit which may be movable relative to the other components of the apparatus.

FIG. 11 illustrates the appearance of strip 12 as it is conveyed through an apparatus 10 such as that illustrated by FIGS. 1 and 2, comprising a first forming apparatus 38, a progressive stamping die 56 and an alignment device 85. The portion of strip 12 shown in FIG. 11 is divided into segments A to G in order to identify the various forming operations according to the invention.

Since the alignment device 85 is located ahead of both the first and second forming apparatus 38, 56 in FIG. 1, the first feature formed in strip 12 is the pilot hole 104. This is illustrated at segment A of the strip 12 shown in FIG. 11. The pilot hole 104 is formed as shown in FIGS. 4 and 8 by the pilot hole punch 96 of the alignment device 85. In order to permit alignment of the strip 12, it is preferred that the progressive stamping die 56 be open during alignment of the strip 12 and formation of pilot holes 104.

Next, at segment B of the strip 12, discontinuities are formed in the central portion of the strip 12 by the first forming device 38. In the preferred embodiment of the invention, the discontinuities comprise a pair of laterally spaced holes 24 and a pair of laterally spaced bosses 26, the center-to-center distance between the holes 24 and the bosses 26 corresponding to a distance L1 and the center-to-center distance between the holes of one plate and the bosses of an adjacent plate corresponding to distance L2.

Following formation of a pilot hole 104 by the pilot hole punch 96, the strip 12 is fed forwardly along the axis S by an incremental amount L2, following which the holes 24 are formed by one of the punches 44 of the first forming device. It is to be noted that the holes 24 are formed with the rollers 30, 32 of the feed mechanism 28 engaging the strip 12. Following formation of holes 24, the strip 12 is advanced by amount L1 and the bosses are formed by one or more of the other punches 44 of the first forming device 38. The bosses 26 are preferably formed in three steps; pre-piercing, forming and final piercing. During formation of the bosses 26, another pilot hole 104 is punched by the alignment device 85. Thus, the strip 12 is advanced by an amount L1+L2 for each alignment operation, so that only one set of pilot holes is punched for each plate.

After formation of the holes 24 and bosses 26, the remaining forming operations, shown at segments C to G of FIG. 11, are performed by the progressive stamping apparatus 56. Preferably, the closing of the progressive stamping die 56 is synchronized with the operation of the alignment device 85 such that the dies of the progressive stamping apparatus 56 close immediately after alignment of the strip 12 by the pilot pin 98. As with the alignment device 85, the strip 12 is advanced by an amount L1+L2 for each progressive stamping operation. The progressive stamping apparatus 56 is preferably provided with index pins 70 which engage the pilot holes 104 and maintain proper alignment of the strip as it passes through the progressive stamping apparatus.

Although the invention has been described in connection with certain preferred embodiments, it is not limited thereto. Rather, the invention includes all embodiments which may fall within the scope of the following claims. 

1. An apparatus for forming a plurality of shaped articles from a strip of material, the apparatus comprising: a feed mechanism for incrementally feeding the strip of material in a forward direction along a strip feed axis; a forming device which receives the strip of material from the feed mechanism and which forms discontinuities in the strip of material at axially-spaced intervals; an alignment device for maintaining constant axial spacing between the discontinuities, the alignment device being located in fixed axial relation to both the forming device and the feed mechanism, the alignment device comprising a frame structure having a first portion and a second portion between which the strip is fed, the first and second portions being movable toward and away from one another, a pilot hole punch for forming pilot holes in the strip and a pilot pin rigidly mounted in axially-spaced relation to one another on the first portion of the frame structure, the pilot pin being spaced forward of the pilot hole punch so that the pilot pin is insertable into a pilot hole previously formed by the punch; wherein the pilot pin and the pilot hole punch each have a length such that when the first and second portions of the frame structure are moved toward one another, the pilot pin passes through a pilot hole before the punch contacts the strip and aligns the strip relative to the pilot hole punch and the forming device.
 2. The apparatus of claim 1, wherein the pilot hole punch and the pilot pin are of substantially the same diameter.
 3. The apparatus of claim 1, wherein the alignment device is located between the feed mechanism and the forming device.
 4. The apparatus of claim 1, wherein the alignment device is located forward or rearward of the forming device.
 5. The apparatus of claim 1, wherein the second portion of the alignment device comprises a first recess for receiving the pilot hole punch and a second recess for receiving the pilot pin.
 6. The apparatus of claim 1, wherein each of the shaped articles has a periphery surrounding a central portion in which the discontinuities are formed, wherein the forming device comprises tooling to permit the formation of discontinuities selected from one or more members of the group consisting of ribs, dimples, apertures and raised apertured bosses.
 7. The apparatus of claim 6, further comprising a stamping apparatus having an upper die portion and a lower die portion, the stamping die forming the peripheries of the articles and separating the articles from the strip.
 8. The apparatus of claim 7, wherein the alignment device is located between the forming device and the stamping apparatus.
 9. The apparatus of claim 7, wherein the alignment device is associated with the stamping apparatus.
 10. The apparatus of claim 7, wherein the stamping apparatus comprises a progressive stamping die having a plurality of stamping stations, including a first stamping station where the strip is received from the forming device.
 11. The apparatus of claim 10, wherein the alignment device is located at the first stamping station.
 12. The apparatus of claim 1, wherein the feed mechanism engages the strip during said incremental feeding and is disengaged from the strip when the first and second portions of the frame structure move toward one another, thereby permitting the alignment device to adjust a position of the strip relative to the feed mechanism.
 13. The apparatus of claim 1, wherein the forming device comprises: a forming frame structure having a first portion and a second portion, the forming frame structure being supported to reciprocate laterally of the strip feed axis; a plurality of punches on the first portion of the forming frame structure; a plurality of dies on the second portion of the forming frame structure, each of the dies being arranged to cooperate with a respective one of the punches and arranged so that the strip can be interposed between the punches and the dies; a punch actuator for actuating each of said punches to form said discontinuities between the punch and its cooperating dies; and a reciprocating drive for reciprocating said frame laterally of said strip feed axis.
 14. The apparatus of claim 1, wherein the pilot pin is longer than the pilot hole punch.
 15. A method for forming a plurality of shaped articles from a strip of material, comprising the following steps: incrementally feeding the strip of material in a forward direction along a strip feed axis; forming discontinuities in the strip of material at axially-spaced intervals; periodically aligning the strip to maintain constant axial spacing between the discontinuities by use of an alignment device comprising a frame structure having a pair of dies between which the strip is fed, at least one of the dies being movable so as to open and close the dies, the alignment device including a pilot hole punch and a pilot pin rigidly mounted in axially-spaced relation to one another on one of the dies; after each said incremental feeding step, closing the dies, thereby causing the pilot hole punch to punch a pilot hole in the strip and to simultaneously cause the pilot pin to pass through a pilot hole previously formed by the pilot hole punch; wherein the pilot pin and the pilot hole punch each have a length such that, during closing of the dies, the pilot pin passes through a pilot hole before the punch contacts the strip, and the pilot pin thereby aligns the strip relative to the pilot hole punch.
 16. The method of claim 15, wherein the incremental feeding step is performed by a feed mechanism which engages the strip during each incremental feeding step and which is disengaged from the strip between the incremental feeding steps.
 17. The method of claim 15, wherein the discontinuities are formed by a forming device comprising a plurality of punches which are reciprocal laterally of the strip feed axis.
 18. The method of claim 17, wherein each of the discontinuities is formed after one of said incremental feeding steps and during engagement of the feed mechanism and the strip. 